1. Field of the Invention
The present invention relates to liquid crystal display apparatuses used as display units for mobile electronic devices such as notebook PCs and PDAs, which are configured with parts and materials that are often affected by external factors such as drop impact, and more particularly to the outer shapes of optical sheets in liquid crystal display apparatuses designed to reduce display failure caused by warpage or deformation due to impact applied to the optical sheets.
2. Background Art
Liquid crystal display apparatuses are being built into a broadening range of electronic devices, particularly portable models, which are becoming ever smaller and lighter.
Liquid crystal display apparatuses are subjected to mechanical and thermal stresses during use. These stresses are often caused by external factors, typically due to dropping and their striking other objects while being carried, and use under diverse environmental conditions. Accordingly, improvement of resistance to these external factors is being demanded. Japanese Patent Unexamined Publication No. 2001-91947 discloses a technology to improve resistance to such external factors.
A conventional liquid crystal display apparatus is described below with reference to drawings.
FIG. 16A is a perspective view of a notebook PC equipped with conventional liquid crystal display apparatus 270. FIG. 16B is a sectional view of FIG. 16A at arrow 16B, and illustrates a structure of conventional liquid crystal display apparatus 270.
As shown in FIG. 16B, liquid crystal display apparatus 270 generally includes light-transmitting liquid crystal panel 271 configured with liquid crystal layer held between a pair of glass substrates and surface light-source unit 272 overlaid on the surface of liquid crystal panel 271. Liquid crystal panel 271 is placed in substantially rectangular resin frame 201. Rectangular metal bezel 273 underlaid on liquid crystal panel 271 is attached to this resin frame 201. Liquid crystal panel 271 is thus interposed and held between resin frame 201 and metal bezel 273.
Normally, surface light-source unit 272 includes light guide plate 274 and tubular light source (fluorescent tube) 275 provided at one end facing light guide plate 274. Optical sheet 202, including multiple sheets such as a diffusing sheet and a prism sheet, is provided at the light-emitting side of light guide plate 274 for controlling the light-emitting characteristic. Reflection sheet 276 is provided at light guide plate 274 at the side opposite the light-emitting face.
Light guide plate 274, optical sheet 202, reflection sheet 276, and tubular light source 275, which configure surface light-source unit 272, are attached to resin frame 201. More specifically, in liquid crystal display apparatus 270, liquid crystal panel 271, resin frame 201, optical sheet 202, light guide plate 274, and reflection sheet 276 are overlaid in this sequence from the light-emitting face. Tubular light source 275 is provided at one end of light guide plate 274, and metal bezel 273 and resin frame 201 sandwich and hold liquid crystal panel 271.
FIG. 17 is a perspective view of conventional liquid crystal display apparatus 270. Only resin frame 201, optical sheet 202, and both-sided tape 204 are illustrated in FIG. 17; no liquid crystal panel, light guide plate, reflection sheet, tubular light source, metal bezel, or driving circuit board are shown.
As shown in FIG. 17, first fixing portion 203 provided at one side of optical sheet 202 is secured onto resin frame 201 with double-sided tape 204. Second fixing portion 205 provided at the other side is not secured with double-sided tape, but fitted to concavity 206 created at resin frame 201.
FIG. 18 is a fragmentary plan view of the details around second fixing portion 205, and illustrates second fixing portion 205 of optical sheet 202 and display area 209 of optical sheet 202. Resin frame 201 and its concavity 206 are also shown. Boundary area 208 between display area 209 of optical sheet 202 and second fixing portion 205 is indicated by the two-dot chain line. As shown in FIG. 18, second fixing portion 205 and concavity 206 position optical sheet 202 onto resin frame 201. More specifically, optical sheet 202 is positioned by making positioning area 207 of second fixing portion 205 contacts the wall of concavity 206.
The operation of conventional liquid crystal display apparatus 270 as configured above is described below.
A thermal stress caused by temperature rise affects optical sheet 202, and a difference in thermal expansion occurs between optical sheet 202 and resin frame 201. Resin frame 201 and first fixing portion 203 are secured with double-sided tape 204, but resin frame 201 and second fixing portion 205 are not secured with the double-sided tape. Accordingly, second fixing portion 205 absorbs the difference in thermal expansion in a longer direction of optical sheet 202. This prevents warpage of optical sheet 202, positional deviation, and so on.
However, if a significant mechanical stress, which is an external factor such as drop impact and vibration, is applied to the conventional structure in direction X, as shown in FIG. 19, optical sheet 202 moves in direction X because concavity 206 created at resin frame 201 is used for positioning, although optical sheet 202 is mostly retained at a predetermined position. As a result, positioning area 207 of second fixing portion 205 may deform or be damaged, as shown in FIG. 19. If this deformation spreads to display area 209 via boundary area 208, display failure such as uneven brightness occurs.